The corrosion of carbon steel (also known as mild steel) is an enormous problem throughout the world. Stainless steels are too costly to use where most structural steel is required so that considerable effort has been spent to find ways of protecting carbon steel from corrosion. Many kinds of treatments, paints and coatings have been developed over the years to address this problem, but those that work well are expensive. For a coating to protect carbon steel successfully against corrosion, it should restrict access of water, oxygen and other oxidants to the steel surface. In some instances, corrosion inhibitors, such as zinc powder or organic amines, are used. The search for new solutions to this problem has continued unabated for several decades.
Recently considerable interest has been shown in the use of electrically conductive polymers in corrosion protection. One of such polymers is poly(aniline) which, when protonated, exhibits very interesting properties as an electrical conductor (specifically, electrical conductivity as high as about 5 S/cm--well into the metallic regime). Experience with this polymer in corrosion prevention, however, has not been encouraging.
In 1981, Mengoli, et al., published a paper in the Journal of Applied Polymer Science, 26, 4247, "Anodic Synthesis of Poly(aniline) Coatings onto Fe Sheets" evaluating anodic synthesis of poly(aniline) coatings onto iron sheets, and concluding that the protection afforded by such coatings, although not negligible, was limited by microporosity and that any interest in such coatings for their protective features would depend upon finding a way to overcome this handicap. Later work by these authors reported in the Journal of Applied Polymer Science, 28, 1125 (1983) evaluated a sulfur-bridged form of poly(aniline) which had limited utility owing to high electrical resistance (i.e., low electrical conductivity), low resistance to chemicals, solvents and aggressive environments.
Musiani, "Improved Poly(aniline) Coatings by in Situ Electropolymerization", Journal of Applied Polymer Science, 29, 4433-38 (1984) confirmed that the coatings described by Mengoli, et al. were inadequate and provided little protection. Further work with electrochemical formation of films from N-(2-hydroxyethyl)aniline showed that the homopolymers made poor coatings because of high porosity and lack of homogeneity but copolymers with a nonionic surfactant gave improved results.
DeBerry, "Modification of the Electrochemical and Corrosion Behavior of Stainless Steels with an Electroactive Coating", Journal of the Electrochemical Society, 132, 1022 (1985) presented a more optimistic view of electroactive poly(aniline) coatings with respect to protection of stainless steel. The coatings evaluated were applied electrochemically using aniline in perchloric acid solution. The author states that PAn in its doped, i.e. conductive, form provides protection to stainless steel in acidic media. Note, however, that stainless steel, intrinsically, is more corrosion resistant than carbon steel. Thus, no direct inference can be made regarding what protection would be afforded to carbon steel by doped PAn. The author further states that a quantitative description of the corrosion protection is not possible due to interactions of variables involved and complexity of the passivation process. On the other hand, Troch-Nagels, "Electron Conducting Organic Coating of Mild Steel by Electropolymerization", Journal of Applied Electrochemistry, 22, 756-764 (1992) points out that on stainless steel, corrosion of the substrate is less compared to mild steel. Electropolymerized poly(aniline) films were found not to be sufficiently effective in providing corrosion protection for mild steel to be of commercial interest. This reference elucidates the theory that electron conducting films could serve as corrosion inhibitors for mild steel. Since corrosion is largely an electrochemical phenomenon, it should take place at the surface of the protecting film without involving the substrate. Only the protonated, doped form of poly(aniline) is said to be electrically conductive (1 to 5 S/cm) and suitable for the research. It was concluded that poly(aniline) films did not meet the requirements for mild steel protection.
A somewhat different approach is reported by Mattson, "The Synthesis of Conducting Polymers for Corrosion Prevention", Final Report on NASA Contract with the University of Central Florida, NASA-NGT-60002, N89-14159, 118-129 (1988). Poly(aniline) was made in an electrically conductive (i.e., doped) form suitable for grinding, sizing and suspension in a binder, such as epoxy or an acrylic latex, for evaluation as a coating to protect mild steel from corrosive environments. Conductivities reported for the doped poly(anilines) prepared were in the range of 0.6 to 10.5 S/cm. Testing of corrosion protection provided by such coatings was not pursued because of flawed samples marked by holes and voids in the coatings. Note that in this study, PAn was used only as a conductive filler, not as a reactive constituent.
More recently, Thompson, et al., "Corrosion-Protective Coatings from Electrically Conductive Polymers", Proceedings from Technology 2001, San Jose, Calif. (1991) reported some success with electrically conductive polymer coatings in the protection of metal surfaces from corrosion. Developed in a joint research effort of NASA at the Kennedy Space Center and the Los Alamos National Laboratory, these coatings involved electrically conductive forms of several polymers, including poly(aniline), which had been appropriately doped with additives serving as electron acceptors to increase conductivity of the polymer. These polymers were used by applying undoped, chemically prepared polymers to the steel surface and subsequently doping the coated surface to the conductive state. Poly(aniline) was determined to show the most promise. Doped poly(aniline) prepared chemically was converted to a nonconducting emeraldine base which was dissolved in an organic solvent and coated on steel, after which the coating was doped to the conductive state. The best dopants were tetracyanoethylene, zinc nitrate and p-toluenesulfonic acid. After doping, a top coat of standard, fully cured epoxy was applied to impart abrasion resistance. Inside R & D, 21, 4 (Jan. 22, 1992) reported this work as providing a corrosion barrier coating for steel made from an electrically conductive polymer which restricts the transfer of electrons from the iron in steel to the oxidizing environment. The coating was said to have two layers: an undercoat of conducting poly(aniline) on the steel and an epoxy top coat for durability. The bilayer coating was reported to be more effective in resisting corrosion than a coating with epoxy alone.
Meanwhile, in a different technology dealing with membrane separations, Huang, et al., "Electrically Conductive Membranes: Synthesis and Applications", Polymeric Materials: Science and Engineering, 61, 895-899 (1989) described the use of poly(aniline) in preparing electrically conductive membranes useful in separating aqueous solutions. Poly(aniline) and polypyrrole were used in chemically doped, electrically conductive forms on substrates of polypropylene and Teflon. The accessibility of the doped membranes to aqueous solutions was said to be greater than for the undoped membranes because of the hydrogen bonding and ionic nature of the doped conductive polymer. This observation was interesting to us because of our work in membrane separations and because it occurred to us that the permeability desired for a separating membrane is the opposite of what is sought in providing a corrosion protective barrier for steel.